1. Field of the Invention
The invention is related to optical systems for monitoring the motion of objects. In particular, increased information bandwidth is achieved in a laser scanning transceiver system through the use of a noncommon collection path for the return light from retroreflectors located on the monitored objects.
2. Description of Prior Art
Wireless monitoring systems, whether based on optical or radio frequency (RF) technologies, are used in many applications where it is desired to monitor the movement of large structures, vehicles or groups of stationary objects. The purpose is to measure the position and/or disturbance to that position of tags located on objects, and to do so with a scanning laser beam that is capable of precisely locating the position and/or change in position of the tags.
Wireless sensor technologies, data platforms, and active identification tags that are based on RF transmission links have become commodity items that are familiar building blocks of monitoring system architectures. These RF communications building blocks offer the ability to provide continuous multiplexed data transfer, on-board data storage with periodic transfers, data transfer on demand (via RF interrogation), and/or periodic state of health verification as may be required to accommodate the needs of various monitoring scenarios. The RF transmissions can be authenticated and encrypted through standardized methods to verify the validity and conceal the content of the transmitted information.
Despite their tremendous versatility, there are applications where the use of RF data transmission systems is prohibited or impractical. The objections arise from the fundamental nature of RF transmissions. Radio waves enable communications by inducing electrical currents in surrounding antennas. The transmitted radio waves must be strong enough to induce a useful current in the antenna of the intended receiver at the desired working distance. Due to the omnidirectional nature of RF transmission, not only is the antenna of the intended receiver excited, but any other conductor that is in the area may be excited as well. At a minimum, this implies that care must be taken with the assignment of transmission frequencies to ensure sufficient channel availability and avoid interaction with other wireless systems. It also suggests the possibility for third-party interception of the data transmissions, which may represent an unacceptable security risk or political irritant. In addition, there are some environments, particularly where explosive atmospheres or high explosives are present, in which RF transmissions may pose a prohibitive physical safety hazard.
All or most of the features of RF transmission links can be implemented with optical links, provided that line-of-sight is maintained between the laser scanner and the object or objects tagged with retroreflectors. The crosstalk, third-party interception, and physical safety limitations of RF transmission links are greatly reduced or totally eliminated when highly directional optical links are used. Optical communication links can replace the RF transmitter and receiver antennas with highly directional optical devices (lasers and retroreflectors) and optical detectors (photodiodes). All of the functional capabilities associated with wireless RF components are still present, and conventional authentication and encryption methods can still be used to encode the transmitted data.
Optical monitoring systems have some unique advantages over RF systems. A typical laser scanning transceiver system is illustrated in FIG. 17. In FIG. 17, a laser scanner has line-of-sight access to the monitored objects or structure. In operation, the scanner can rapidly scan pre-programmed points corresponding to the positions of cooperative targets (retroreflectors) affixed to objects of interest. The security of an object is verified by determining that the retroreflector is still present, and that its position has not changed.
Laser scanning transceivers are usually connected to a computer which maintains selected pertinent data regarding the retroreflectors. The angular position of each retroreflector relative to the scanner is stored, and represents the position where the transceiver will point the scanning beam to look for a reflected return signal.
In its simplest form, if the scanner receives a return signal from a retroreflector, and if it is of similar intensity to a stored intensity level associated with that object's retroreflector, the indication is that the object is still present. This is sufficient information when rapidly scanning an entire array of objects to determine that each is present. The laser scanner output is aligned to the receiver input during initial setup and beyond that, the system operation is functionally equivalent to that of the RF systems, with multiple optical wavelengths taking the place of multiple RF frequencies.
A feature of laser scanning systems is that remote third party interception of the optical transmissions can become virtually impossible because interception of the transmissions require some physical access to the line-of-sight optical beam between the transmitter and receiver. This line-of-sight property can also reduce the need for complex encryption schemes because the scanning process would quickly detect tampering with the optical link.
In some applications such as monitoring stored high value objects, better target (retroreflector) identity is needed, and improved integrity verification of the scans is also desired. This requires that more detailed information be stored for each target. When a target is added to the list of monitored targets and installed in a location for scanning, a detailed scan of the target is performed. The resulting high-resolution intensity data map is stored for comparison with later detailed scans to uniquely identify the target. These data are also used for verification of target integrity because small translational or rotational displacements of the retroreflector are readily identifiable.
As object security becomes ever more important, even better means for ensuring the integrity of scanning laser systems is needed. We realized some possible ways of doing this might be by altering the scanning laser beam by the use of amplitude or frequency modulation, polarization, change of beam shape or other such means. These kinds of beam alterations are not known in laser scanning transceiver systems, but are realized by our invention. Laser scanning transceiver systems are usually configured such that the return beam from the retroreflector element lies on the same axis as the incident beam from the laser. Any change imposed on the incident laser beam such as modulation or polarization would be returned by the retroreflector, and would mask the modulation or polarization induced changes in the return beam that one wants to detect. Some examples of the prior art follow.